Solar cells have been developed as clean, renewable energy sources to meet growing demand. Currently, crystalline silicon solar cells (both single crystal and polycrystalline) are the dominant technologies in the market. Crystalline silicon solar cells must use a thick substrate (>100 um) of silicon to absorb the sunlight since it has an indirect band gap. Also, the absorption coefficient is low for crystalline silicon because of the indirect band gap. The use of a thick substrate also means that the crystalline silicon solar cells must use high quality material to provide long carrier lifetimes to allow the carriers to diffuse to the contacts. Therefore, crystalline silicon solar cell technologies lead to increased costs. Thin film solar cells based on amorphous silicon (a-Si), CIGS, CdTe, CZTS, etc. provide an opportunity to increase the material utilization since only thin films (<10 um) are generally required. CdTe and CZTS films have band gaps of about 1.5 eV and therefore, are efficient absorbers for wavelengths shorter than about 800 nm. The absorption coefficient for CdTe is about 105/cm and the absorption coefficient for CZTS is about 104/cm. CIGS films have bandgaps in the range of 1.0 eV (CIS) to 1.65 eV (CGS) and are also efficient absorbers across the entire visible spectrum. The absorption coefficient for CIGS is about 105/cm. Furthermore, thin film solar cells may be fabricated on inexpensive substrates such as glass, plastics, and thin sheets of metal. Among the thin film solar cells, CIGS has demonstrated the best lab cell efficiency (over 20%) and the best large area module efficiency (>12%).
The increasing demand for environmentally friendly, sustainable and renewable energy sources is driving the development of large area, thin film photovoltaic (TFPV) devices. With a long-term goal of providing a significant percentage of global energy demand, there is a concomitant need for Earth-abundant, high conversion efficiency materials for use in photovoltaic devices. A number of Earth abundant direct-bandgap semiconductor materials now seem to show evidence of the potential for both high efficiency and low cost in Very Large Scale (VLS) production (e.g. greater than 100 gigawatt (GW)), yet relatively little attention has been devoted to their development and characterization.
Among the TFPV technologies, CIGS and CdTe are the two that have reached volume production with greater than 10% stabilized module efficiencies. Solar cell production volume must increase tremendously in the coming decades to meet sharply growing energy needs. However, the supply of In, Ga and Te may impact annual production of CIGS and CdTe solar panels. Moreover, price increases and supply constraints in In and Ga could result from the aggregate demand for these materials used in flat panel displays (FPD) and light-emitting diodes (LED) along with CIGS TFPV. Also, there are concerns about the toxicity of Cd throughout the lifecycle of the CdTe TFPV solar modules. Efforts to develop devices that leverage manufacturing and R&D infrastructure related to TFPV using more widely available and more environmentally friendly raw materials should be considered a top priority for research.
The immaturity of TFPV devices exploiting Earth abundant materials represents a daunting challenge in terms of the time-to-commercialization. That same immaturity also suggests an enticing opportunity for breakthrough discoveries. A quaternary system such as CIGS or CZTS requires management of multiple kinetic pathways, thermodynamic phase equilibrium considerations, defect chemistries, and interfacial control. The vast phase-space to be managed includes deposition, and conversion methods, process parameters, source material choices, compositions, and overall integration schemes. Traditional R&D methods are ill-equipped to address such complexity, and the traditionally slow pace of R&D could limit any new material from reaching industrial relevance when having to compete with the incrementally improving performance of already established TFPV fabrication lines.
However, due to the complexity of the material, cell structure and manufacturing process, both the fundamental scientific understanding and large scale manufacturability are yet to be improved for CIGS and CZTS solar cells. As the photovoltaic industry pushes to achieve grid parity, much faster and broader investigation is needed to explore the material, device, and process windows for higher efficiency and a lower cost of manufacturing process. Efficient methods for forming different types of CIGS and CZTS solar cells that can be evaluated are necessary.
The TCO materials for TFPV solar cells must meet a number of requirements such as high conductivity, good ohmic contact to neighboring layers, good adhesion, thermo-mechanically stable during manufacturing and during deployment in the field, low cost, high deposition rate, a coefficient of thermal expansion preferably similar to the substrate and other materials in the TFPV solar cell stack, resistance to moisture, etc. It is difficult to develop a single material that will meet all of these requirements.
Therefore, there is a need to develop TCO materials that address each of the requirements listed above to improve the efficiency of TFPV solar cells. In addition to meeting the required materials properties, the TCO material must also meet the requirements of low cost and ease of manufacturing.